1. Field of the Invention
The present invention relates to a fiberoptic block used as optical image transmitting means in a fingerprint acquisition apparatus or the like.
2. Related Background Art
FIG. 11A is a side view showing a conventional fiberoptic block (referred to as "FOB" hereinafter) 50, whereas Fig. 11B is an enlarged partially sectional view showing a configuration of the FOB 50. This FOB 50 is typically used as irregularity image transmitting means of an apparatus (e.g., fingerprint acquisition apparatus) for detecting an irregularity image formed by irregularities of an object surface.
As shown in FIG. 11A, the FOB 50 has a configuration in which a plurality of optical fibers 56 are bundled together such that their axes are substantially in parallel to each other. These optical fibers, each of which will be referred to as a unit fiber hereinafter, are positioned such that their both end faces are substantially made flush with each other, respectively. Input and output end faces 52 and 54 are respectively used for inputting and outputting an optical image. These end faces, which are in parallel to each other, are respectively formed by thus collected both end faces of the unit fibers. Also, the input and output end faces 52 and 54 are inclined with respect to the axial direction of the unit fibers by an angle .alpha.. When this FOB 50 is utilized for an irregularity image detecting apparatus, a subject such as a finger is placed on the input end face 52.
As shown in FIG. 11B, each unit fiber 56 comprises a core 60, in its center, serving as a light propagating region; a cladding 61 closely surrounding the core 60; and a light absorber 62 closely surrounding the cladding 61. Both end faces of each unit fiber are inclined with respect to its axis 64 by the angle .alpha.. In other words, both end faces of each unit fiber are inclined such that the angle formed between a normal of these end faces and the axis 64 becomes (90.degree.-.alpha.). The angle of inclination (slant angle) .alpha. is set to an angle at which, when light is incident on the core 60 from within the air, the incident light is not totally reflected by the interface between the core 60 and the cladding 61. In other words, the slant angle .alpha. is set to an angle range in which the angle of incidence of light incident on the core 60 from within the air with respect to the interface between the core and the cladding is not greater than the critical angle at this interface.
As is well-known, such a range of slant angle .alpha. can be expressed as .alpha..ltoreq..alpha..sub.c wherein .alpha..sub.c is a specific angle. Here, .alpha..sub.c is an angle satisfying the following three equations:
n.sub.0 .multidot.sin .theta..sub.c =n.sub.1 .multidot.sin 90.degree. (total reflection condition between the core and the cladding) PA1 n.sub.0 .multidot.sin .beta.=n.sub.a .multidot.sin 90.degree. (law of refraction between the air and the core) PA1 .alpha..sub.c +(90.degree.+.beta.)+(90.degree.-.theta..sub.c)=180.degree. (sum of interior angles of a triangle)
In the above equations, n.sub.0 is a refractive index of the core 60, n.sub.1 is a refractive index of the cladding, and n.sub.a is a refractive index of the air. Also, .theta..sub.c is a critical angle at the interface between the core and the cladding, .beta. is an angle formed between a normal of the input end face 52 and light (indicated by its corresponding arrow in FIG. 11B) which is incident on the input end face 52 with an incident angle of 90.degree., i.e., angle of refraction of the incident light with an incident angle of 90.degree..
When .theta..sub.c and .beta. are eliminated from the above three equations so as to determine .alpha..sub.c the above-mentioned range of slant angle .alpha. is expressed as: EQU .alpha..ltoreq..alpha..sub.c =sin.sup.-1 (n.sub.1 /n.sub.0)-sin.sup.-1 (n.sub.a /n.sub.0) (1)
In the case where, while a subject is placed on the input end face 52, the contact surface of the subject with respect to the input end face 52 is irradiated with illuminating light, a light component made incident on the core 60 of a unit fiber by way of a protruded portion in irregularities of the subject surface which is in contact with the input end face 52 is propagated through the core 60, while being totally reflected by the interface between the core and the cladding, so as to exit from the output end face 54. On the other hand, a light component made incident on the core 60 by way of a depressed portion of the subject surface enters the core 60 after passing through an air layer which exists between the subject surface and the input end face 52. Since the input end face 52 of the FOB 50 is inclined at a slant angle which is within the angle range mentioned above, the latter incident light component is not totally reflected by the interface between the core and the cladding, whereby a part thereof leaks into the cladding 61. In this manner, whenever the incident light from the depressed portion of the subject reaches the interface between the core and the cladding, it partially leaks into the cladding 61 and then is absorbed by the light absorber 62. Consequently, the incident light is gradually attenuated as it advances, thereby failing to reach the output end face 54. Accordingly, only the light component made incident on the input end face 52 by way of the protruded portions of the subject surface can substantially be emitted from the output end face 54. As a result, a bright and dark image with a high contrast corresponding to the irregularities of the subject surface can be obtained.
In the foregoing manner, a bright and dark image (irregularity image) corresponding to the irregularities of the subject surface is transmitted by the FOB 50 so as to be emitted from the output end face 54. Accordingly, as shown in FIG. 12, when the output end face 54 of the FOB 50 is attached to a photodetector (CCD detector 70 in FIG. 12) so as to be butted against the input face of the photodetector (input face 74 of a CCD chip 72 in FIG. 12), an irregularity image of the subject surface can be detected.
In the conventional FOB 50, the output end face 54 is inclined with respect to the axis 64 of the unit fiber. Consequently, when the medium outside the output end face 54 is a medium, such as the air, having a refractive index lower than that of the core 60, total reflection may occur between this medium and the core 60. That is, the light incident on the input end face 52 from the protruded portions of the subject may not be emitted from the output end face 54. Accordingly, in the conventional FOB 50, it is often the case that a sufficiently bright irregularity image is obtained only when the gap between the output end face 54 and the input face 74 of the CCD detector 70 is filled with a matching liquid, a refractive-index-matching adhesive, or the like.
Also, in the conventional FOB 50, since the irregularity image of the subject is substantially emitted along the axis 64 of the unit fiber, it is not emitted perpendicularly to the output end face 54. Accordingly, the irregularity image of the subject is not perpendicularly made incident on the input face 74 of the CCD detector 70, whereby the irregularity image detected by the CCD detector 70 may become dark as a whole.
Further, since the slant angle of the input end face 52 set as mentioned above is typically very small, the cross section of the FOB 50 taken along a plane including the axis 64 of each unit fiber becomes a parallelogram in which lateral sides are greatly inclined with respect to a normal of its base. As a result, the conventional FOB 50 tends to have a larger width, thereby making it difficult to be attached to an input recess 76 of the CCD detector 70. Moreover, since the side faces of the FOB 50 are greatly inclined with respect to the normal of its bottom face, even after being attached to the CCD detector 70, an edge of the FOB 50 is likely to project to a side of the CCD detector 70, for example, whereby a complex device comprising the FOB and the CCD detector tends to become bulky.
In view of the foregoing, it is an object of the present invention to provide a fiberoptic plate which can output a bright irregularity image, can be easily attached to a photodetector, and can keep a compact size even after being attached thereto.